Group III-V semiconductors are increasingly being used in light-emitting diodes (LEDs) and laser diodes (LDs). Specific III-V semiconductors, such as gallium nitride (GaN), are emerging as important materials for the production of shorter wavelength LEDs and LDs, including blue and ultra-violet emitting optical and optoelectronic devices. Thus, there is increasing interest in the development of fabrication processes to make low-cost, high-quality III-V semiconductor films.
One widely used process for making III-V nitride films like GaN is hydride vapor-phase epitaxy (HVPE). This process includes a high-temperature, vapor-phase reaction between gallium chloride (GaCl) and ammonia (NH3) at a substrate deposition surface. The GaCl precursor is produced by passing hydrogen chloride (HCl) gas over a heated, liquid gallium supply (melting point 29.8° C.). The ammonia may be supplied from a standard gas source. The precursors are brought together at the heated substrate, where they react and deposit a layer of GaN. The HVPE deposition rate is high (e.g., up to 100 μm/hr) and provides a relatively fast and cost effective method of making GaN films.
However, the HVPE also has drawbacks for forming GaN and other III-V compound films. The HCl gas is not completely consumed when forming the GaCl, and the substrate is exposed to significant amounts of HCl during film deposition. For substrates like silicon that are etch-sensitive towards HCl, a pre-film anti-etch layer needs to be deposited to protect the substrate from being damaged or destroyed. The additional layer needs to be carefully selected so that it minimally interferes with the formation of the GaN film. At the very least, the formation of the anti-etch layer will add additional cost and time to the GaN film deposition process.
In addition, the high deposition rates that characterize HVPE processes make them difficult to use with low levels of dopant materials. Dopants are often important to define the electrical and optoelectronic properties of a III-V compound LED, LD, transistor, etc. Doping steps done after the GaN film is deposited may not provide an adequate concentration or homogeneity of the dopant in the film. When post-deposition doping is possible at all, it will at the very least add additional cost and time to the GaN film deposition process.
Another major drawback of HVPE is the difficulty of using the process to grow alloys of III-V nitrides, such as aluminum gallium nitride (AlGaN) and indium gallium nitride (InGaN). These and other nitride alloys offer a much larger variety of heterostructures than single-metal nitrides, and are already suggesting many new optoelectronic device applications. But unfortunately generating stable gas precursors for aluminum (e.g., aluminum chloride) and indium (e.g., indium chloride) has proven more difficult than the generation of GaCl.
For example, aluminum has a much higher melting point (about 660° C.) than gallium, and the chloride salt of aluminum (AlCl3) quickly solidifies into a low vapor pressure solid even under high-temperature HVPE reactor conditions. When HCl passes over aluminum metal, most of the AlCl3 precipitates out of the gas flow, and only a small fraction reaches the deposition substrate to react with a nitrogen precursor and form AlN.
To overcome these and other shortcomings of HVPE III-V compound film formation, another process called metal-organic chemical vapor deposition (MOCVD) is used to form III-V nitride films. MOCVD uses a reasonably volatile metallorganic Group III precursor such as trimethylgallium (TMGa) or trimethylaluminum (TMAl) to deliver the Group III metal to the substrate where it reacts with the nitrogen precursor (e.g., ammonium) to form the III-V nitride film.
MOCVD nitride films are typically deposited at lower temperature than HVPE films, allowing the fabrication process to have a lower thermal budget. It is also easier to combine two or more different Group III metallorganic precursors (e.g., Ga, Al, In, etc.) and make alloy films of GaN (e.g., AlGaN, InGaN, etc.). Dopants may also be more easily combined with the precursors to deposit an in-situ doped film layer.
MOCVD film depositions, however, also have drawbacks. These include slower deposition rates for MOCVD than HVPE. MOCVD typically deposits a film at about 5 μm/hr or less compared with 50 μm/hr for HVPE. The slower deposition times make MOCVD a lower throughput and more expensive deposition process than HVPE.
Several approaches have been tried to increase the throughput of GaN depositions with MOCVD: In one approach, batch reactors have been tried that are capable of simultaneously growing films on many wafers or over large areas. In a second approach, attempts were made to increase the rate of GaN film growth and heterostructures. Both approaches have had difficulties.
Scale up to large areas has proved difficult because the GaN must be grown at relatively high pressures (e.g., several hundred Torr), and at these pressures the flow velocity in a large reactor is low, unless the total flow through the reaction is made extraordinarily high. Consequently, the precursor stream becomes depleted of reactants over a short distance, making it difficult to grow a uniform film over a large area.
Attempts to increase the deposition rates of a GaN film by increasing the concentration (i.e., partial pressures) of the organo-gallium and ammonia precursors have also proved difficult. FIG. 1A shows a graph of a growth rate for a GaN film as a function of the total pressure in the MOCVD reactor. These graphs are based on simulations by STR of GaN film growth in a Thomas Swan reactor with a close-coupled showerhead injector. The graph shows a steep drop in the rate as the pressure in the reactor increases above about 300 torr.
The decrease in GaN film growth rate with increasing MOCVD reactor pressure is attributed to the formation of gas-phase parasitic particles that consume the Ga and N precursors that would otherwise be used to grow the film. These parasitic particles form in a thin thermal boundary layer over the wafer substrate, where local gas temperatures become sufficiently high to promote a pyrolytic reaction between the Group III precursors and ammonia (the nitrogen precursor). Once formed, the hot, suspended (by thermophoresis) particles become nuclei for additional deposition, thereby growing and further depleating reactants from the gas stream, until they are flushed out of the chamber. Thus, there is competition between the desired film growth and the parasitic particle growth. Parasitic particle formation increases when the partial pressures of the Group III and/or Group V precursors increase, or when the thermal boundary layer around the wafer substrate is expanded.
In the case of GaN films grown with a trimethylgallium precursor, the film growth rate eventually saturates with respect to the trimethylgallium flow, making it difficult to realize growth rates greater than about 5 μm/hr. The formation of the parasitic particles can also degrade the optoelectronic qualities of the deposited GaN film.
Because the parasitic particle formation depends on the partial pressures of the Group III and V precursors, it may be possible to increase the growth rate of the MOCVD deposited film by diluting the precursor gas stream with more carrier gas (e.g., hydrogen (H2), helium, etc.). However, attempts to dilute the precursor gas stream hurt the quality of the III-V film that was deposited. Maintaining high partial pressures of the precursors, especially a high ammonia partial pressure in the case of nitride film depositions, appears to be beneficial in the growth of high quality films.
Parasitic particle formation in MOCVD film depositions can be even more severe for alloys of gallium nitride. FIG. 1B, for example, shows a graph of a STR simulation of the deposition rate of AlGaN as a function of the pressure in an Aixtron planetary reactor. The graph shows an even steeper drop off in the film formation rate versus reactor pressure during the formation of a AlGaN film than for an unalloyed GaN film. Similar decreases in film growth rates were shown in simulations for Thomas Swan and Veeco reactor geometries.
AlGaN films are used in LED heterostructures where a p-type layer is grown over a InGaN well active region. It is therefore beneficial to grow the AlGaN film with a reasonably high hole concentration, and free of nanradiative or compensating defects. Unfortunately, high total pressures and high ammonia flows are best for growing AlGaN films with these qualities, but growing these films with the requisite Al content by MOCVD is extremely challenging due to the formation of the parasitic particles.
In another example, InGaN film growth is also limited by parasitic particle formation. FIG. 1C shows a graph of an InGaN film growth rate as a function of reaction pressure. The graph was derived from growth simulation done with a Thomas Swan showerhead reactor geometry at various pressures. While the formation of parasitic particles in MOCVD depositions of InGaN is not as pronounced as for AlGaN, it is still significant enough to limit the growth rate of the films. InGaN films have applications in the quantum well active regions of laser diodes and LEDs. Without the formation of the parasitic particles, growth of InGaN films could be performed at higher pressures and higher ammonia flow, both of which would be beneficial for the optoelectronic quality (e.g., high internal efficiency) and p-type doping in LDs and LEDs. Thus, there is a need for systems and methods that control parasitic particle formation while increasing the throughput of MOCVD formed III-V nitride films.